The invention concerns a method and a device for measuring the chromatic dispersion of an optical transmission link.
Since the refractive index of optical glass fibers depends on the wavelength, different propagation velocities are obtained for signals with different wavelengths. This may result in the spectral components of a modulated optical signal not arriving simultaneously at the far end, and in signal distortions.
The group delay time tg is used as a measure for the propagation time of a signal component. The derivation of the group delay time tg by the wavelength xcex referred to the length L of the fibers is called chromatic dispersion coefficient D: D=1/L*dtg/dxcex.
The measuring methods commonly used (xe2x80x9cModulation Phase-Shift Methodxe2x80x9d, xe2x80x9cDifferential Phase-shift Methodxe2x80x9d) are based on the classical Nyquist method for measuring the group delay time. The optical carrier signal is amplitude modulated with a frequency xcfx89, which is small compared to the carrier frequency. In the 1550 nm range the optical frequencies are at about 200 THz, such that this requirement is met for modulation frequencies up to the high GHz range. The modulation frequency is recovered at the end of the test sample by demodulation and its phase position is compared to that of the modulation source. According to Nyquist the group delay time is then obtained as : tg=xcex94xcex2/xcfx89.
For determining the chromatic dispersion, the group delay time must be determined in dependence on the wavelength and then the derivation of the group delay time by the wavelength must be calculated.
In practice, one often works with few discrete wavelengths, such that the chromatic dispersion is only determined approximately. The required number and distance of the wavelengths depends on the test sample. For glass fibers relatively coarse wavelength steps of about 5 or 10 nm are sufficient. For narrowband test samples, for example dispersion compensators with chirped fiber Bragg gratings, significantly smaller step sizes are necessary (for example  less than 0.5 nm).
The classical measurement setup or the Nyquist method uses a tuneable laser source and a downstream (external) modulator. As modulation frequencies one finds values in the range 50 MHz to several GHz, which again makes necessary a powerful high frequency source and a broadband photo detector. The entire kit thus becomes relatively expensive. Furthermore, such setups are not suitable for end-to-end measurements, since the modulation signal on the sending side must be fed to the evaluation side as a reference. In known solutions this occurs electrically or optically via an additional cable connection.
The object of the present invention is to provide a method and an apparatus for measuring the chromatic dispersion of an optical transmission link without additional reference connection.
The aforementioned object is solved according to the invention by a method for measuring the chromatic dispersion of an optical transmission link, preferably of a light waveguide, wherein an amplitude modulated broadband optical signal is fed into the optical transmission link to be measured, wherein the transmitted signal is separated into a reference path, in which the modulation frequency is demodulated out of the transmitted signal, and into a measuring path, in which the transmitted signal is demodulated at several optical frequencies respectively, and wherein the chromatic dispersion of the optical transmission link is determined from the phase differences between the several demodulated signals of the measuring path and the demodulated signal of the reference path.
According to the invention an optical broadband source can, for example, be provided on the sending side, which source is amplitude modulated with a high frequency oscillation. The transmitted optical signal is separated at the receiving side as a reference signal into a reference path and as measurement signal into a measuring path. The reference signal passes through a broadband optical filter, and then the modulation signal is recovered with a photo detector, which modulation signal serves as a reference for a phase indication. The measuring signal passes through a tuneable narrowband optical filter, and there also, the modulation signal is recovered with a photo detector, wherein the phase position of the modulation signal differs from the reference signal due to chromatic dispersion. From the change of the phase difference at different positions of the narrow band optical filter, the chromatic dispersion is then determined.
The individual measuring signals obtained at the several optical frequencies are each significantly weaker than the reference signal. This signal difference can be reduced by coupling the transmitted signal for the most part, preferably at least 80%, into the measuring path.
Preferably, for measuring the phase difference, the demodulated signals are first transferred to a lower frequency range.
The aforementioned object is also solved according to the invention by an apparatus for measuring the chromatic dispersion in an optical transmission link, preferably of a light waveguide, with an amplitude modulated, broadband light source at the input side of the optical transmission link to be measured, with a device which separates the transmitted optical signal into a reference path and a measuring path, with a tuneable narrowband optical filter provided in the measuring path, with a photo detector in the reference path and in the measuring path respectively and with a phase measuring device provided at the output side of the two photo detectors for determining the phase differences respectively present between the reference and measuring path, from which the chromatic dispersion of the optical transmission link can be determined. The tuneable narrowband optical filter is disposed in the measuring path in order to be able to analyze the measuring signal in dependence on the wavelength. For this purpose, the bandwidth of the light source should be at least one magnitude larger than the respective filter bandwidth of the tuneable optical filter.
In the reference path a broadband optical filter may be required upstream of the photo detector in order to constrict the optical bandwidth of the transmitted signal such that cancelling out of the modulation signal due to chromatic dispersion is avoided. In practice, the bandwidth of the broadband optical filter could, for example, be 30 nm. In many cases such a broadband optical filter is not necessary if the amplitude modulated light source is suitably chosen (for example a light source with a limited bandwidth).
Preferably, a frequency source is connected to the reference path and the measuring path, which transfers the demodulated signals of both paths, for example 20 MHz signals, into a lower frequency range by frequency mixing, for example into the kHz range. Theoretically, the requirements for the phase sensitivity of the frequency source are small, since its influence is effective in the reference path and the measuring path and is cancelled out during phase measurement. The transferred signals are amplified and constricted, such that the amplitude of the measuring signal does not enter the measuring result.
Known phase meters only have a limited measuring range of, for example, 360xc2x0. For larger phase values the output voltage of the phase meter jumps back, for example to the value at 0xc2x0, in order to rise again to its maximum value at 720xc2x0 etc. This ambiguous behaviour with irregularities in the phase meter characteristic aggravates the analysis. In preferred embodiments of the inventive measuring apparatus the phase measuring device therefore comprises two phase meters, the characteristic curves of which are offset to one another, preferably by 180xc2x0. This arrangement with two phase meters operating at an offset has the advantage that always at least one phase meter operates at a safe distance of such an irregularity. This is particularly important, if the signals to be analyzed are disturbed by noise. Since only small phase differences and small wavelength differences are analyzed for determining the chromatic dispersion, a phase meter with a large continuous measuring range is also not necessary. Instead, the proposed arrangement of two phase meters with limited measuring range operating at an offset obtain a higher resolution and measuring accuracy.
Additional advantages of the invention can be gathered from the description and the drawing. Also, the previously mentioned and the following characteristics can be used according to the invention each individually or collectively in any combination. The embodiments shown and described are not to be taken as a conclusive enumeration, but have exemplary character for the description of the invention.